1. Field of the Invention
This invention relates to a gas sensor using titanium oxide (TiO.sub.2)as a sensitive member and to a method of producing the gas sensor. More particularly, the present invention relates to a gas sensor having a layer which is used as a sensor sensitive member and has a lot of fine voids formed in the surface, and to a method of producing the gas sensor.
2. Description of the Prior Art
A gas sensor measures the concentration of oxygen, hydrocarbons, alcohols, fumes, humidity, and the like, in an atmosphere. Generally, the gas sensor has a sensitive member and a pair of electrodes on an electrically insulating substrate. A part of the surface of the sensitive member is sometimes covered with a protective film. Titanium oxide (TiO.sub.2) is known as one of the materials for the sensitive member.
Titanium oxide is an n-type semiconductor. This titanium oxide has a property such that its resistance value increases with an increasing oxygen concentration in an atmospheric gas. Accordingly, a proposal has been made to use titanium oxide as the sensitive member of the gas sensor by utilizing this property.
An oxygen sensor is disclosed in Japanese Patent Laid-Open No. 164948/1981 as an example of the conventional titanium oxide type sensors. The oxygen sensor is produced by adding 0.5 to 20 mol % of silicon oxide to 99.5 to 80 mol % of titanium dioxide, adding an organic solvent to the mixture to turn it to a slurry, shaping the slurry to a sheet and putting electrodes on both surfaces of the sheet.
However, this gas sensor is a sintered product and impurities from silicon dioxide or from the organic solvent will mix, so that the response of the sensor is slow. It is known, on the other hand, how to improve the response of the gas sensor by lowering the sintering temperature thereby making the sensitive member porous in order to improve the response. However, this method involves another problem in that it reduces the mechanical strength of the sensitive member. When a sensor produced by this method is used for measuring the concentration of an exhaust gas of automobiles or for controlling the combustion of heating equipment, the durability of the gas sensor is not sufficient because of thermal stress in the thermal cycle during repeated use of the sensor at temperatures ranging from normal temperature to close to 1,100.degree. C. and the stress due to mechanical vibration.
To improve the strength and response of the gas sensor, an invention is disclosed in Japanese Patent Laid-Open No. 124059/1980 in which an electrode--metal oxide (as the sensitive member)--electrode--protective film are sequentially disposed on a heat-resistant insulating substrate, and titanium dioxide is used as the metal oxide. This invention makes it possible to reduce the thickness of the sensitive member because it is formed by screen printing, and to improve the response by reducing the diffusion distance of the detected gas inside the sensitive member. However, the problem of the mixture of impurities from the organic solvent remains unsolved. Moreover, this prior art invention plasmasprays spinel such s MgO.Al.sub.2 O.sub.3 as a gas permeable protective film on the surface of the sensitive member in order to improve the durability or response of the sensor.
To improve the response of an oxygen sensor element consisting of titanium dioxide, Japanese Patent Laid-Open No. 41764/1980 discloses a sensor in which 0.02 to 20 mol % of the elements of the Group V of the Periodic Table such as antimony (Sb), niobium (Nb), tantalum (Ta) and the like and 0.02 to 20 mol % of platinum (Pt) are added to a sintered body consisting of titanium dioxide as the principal component. This sensor, too, is a sintered body. The durability of the sensor can be improved by elevating the sintering temperature in the sintering process, but the response is reduced. If the sintering temperature is lowered, the response can be improved, but the durability will be reduced and the problem of mixture of impurities will occur.
If Sb, Nb, Ta or the like is added to titanium dioxide, the internal resistance of the sensor will change and its response will be reduced in the course of use of the sensor for an extended period.
Japanese Patent Laid-Open No. 124058/1980 discloses another type of sensor. In accordance with this prior art, titanium dioxide used is in the form of particles having a maximum size of 100.ANG. which are precipitated as a gel-like aqueous solution to form a flake-like anatase structure. The sensor is shaped from this structure in such a fashion that its surface area is at least 3 m.sup.2 /g. The sensor of this prior art is not substantially different from the sensors of the type described already in its response and durability brought forth by sintering , but since the sensor is composed of only the sensitive member, its mechanical strength is low.
Japanese Patent Laid-Open No. 140138/1980 discloses an invention in which the titanium oxide sintered body is a porous body having a relative density of from 0.7 to 0.5, the average particle size of titanium oxide is from 1 to 5 .mu.m and partial fusion is caused between the particles to improve the response. Since the titanium oxide sintered body is produced by the greensand method, this prior invention has low durability. When the sensitive member is porous as a whole and is disposed on an insulating substrate, the diffusion distance in the adsorption and desorption of a detected gas becomes great. Therefore, when a gas concentration changes abruptly, tailing occurs. To minimize tailing, Japanese Patent Laid-Open No. 18922/1980 discloses an invention in which a 50 to 100 .mu.m thick powdery paste containing a transition metal oxide is deposited by screen printing on an insulating substrate made of ceramics, and after sintering, MgO and Al.sub.2 O.sub.3 are further coated thereon. In this case, too, the problems of the mixture of impurities from the paste and the tailing phenomenon resulting from MgO.Al.sub.2 O.sub.3 coating remain unsolved.
As described above, gas sensors using metal oxides, particularly titanium oxide, have been well known. All the known titanium oxide type gas sensors covert fine powder as the raw material to a slurry, and sinter the slurry to form the sensitive member. The response of the titanium oxide gas sensor becomes higher with a decreasing particle diameter of the starting fine particles. However, when the sensitive member is molded by sintering as in the prior art described above, the fine particles of titanium oxide are mutually diffused and fused in the subsequent sintering process, thereby forming the sensitive member. The mechanical strength of the resulting sensitive member is significantly affected by the area of this interdiffusion. The area of interdiffusion becomes greater with a higher sintering temperature, and the strength can be improved, too. As this area becomes greater, however, the neck portions become smaller which cause the resistance change due to the adsorption and desorption of the gas, so that the response is reduced. If a trace element is added in order to promote sintering and thus to improve the response, the resistance value of the sensitive member will change with time in the course of a long period so that the gas sensing temperature will drop.
A method which sinters a gas sensitive member on a ceramic substrate has also been known. In this case, the state of close contact of the gas sensitive member with the substrate becomes a factor that determines the durability of the sensitive member. To improve the close contact and strength between titanium dioxide and the ceramic substrate, it is desired that interdiffusion be caused between the ceramic substrate and titanium dioxide. In accordance with the conventional sintering method, however, the sintering temperature must be raised as described already, but interdiffusion of the sensitive member develops with the temperature rise and with the progress of the reaction, so that the response drops eventually.